The present invention generally relates to display units and, more particularly, to a display unit which can automatically test display data.
Recently, computer graphic output units have been proposed to output various graphics. In most cases, a cathode ray tube (CRT) such as a television monitor is used as the graphic output unit.
On the other hand, in order to make an accurate data display, it is necessary to test whether or not the input data are correctly displayed on the CRT.
Accordingly, there are demands to realize a method of accurately and efficiently testing the display unit.
Generally, a CRT display unit of a work station or the like uses a memory which is often referred to as a frame buffer for temporarily storing display data. FIG.1 shows an example of such a CRT display unit.
A CRT display unit 1 shown in FIG. 1 includes a frame buffer 2, a central processing unit (CPU) 4, a display controller 7, and a CRT 8.
The frame buffer 2 has two input/output ports and is formed by a pair of memories which are mutually unaffected. In other words, the frame buffer 2 is a so-called dual port random access memory (RAM).
One input/output port of the frame buffer 2 is called a RAM port 3. The data can be read from arbitrary addresses via this RAM port 3. In addition, the RAM port 3 is connected to a bus 5 of the CPU 4, and the display data can be written via this RAM port 3.
The other input/output port of the frame buffer 2 is called a serial access memory (SAM) port 6. The data can be read from and written to consecutive addresses via this SAM port 6, and the SAM port 6 is used for sending the data to the display controller 7. The display controller 7 drives and controls the CRT 8.
Conventionally, when carrying out a read/write test with respect to the display unit 1 using the frame buffer 2 which is formed by two independent memories, the test in general was carried out by performing a data write/read operation of the frame buffer (dual port RAM) 2.
However, in the display unit 1 which uses the frame buffer 2, there is no guarantee that the data are accurately displayed even if the written data are read out correctly because the RAM and the SAM are independent. In other words, according to the conventional display unit 1, it was impossible to directly compare the data input to the RAM port 3 and the data output to the SAM port 6.
For this reason, in order to carry out the read/write test, the only way was to write a specific pattern in the frame buffer 2 and to display the specific pattern on the CRT 8 so that the operator can check to see if the displayed specific pattern matches the specific pattern written into the frame buffer 2. This was the only way of obtaining accurate information, and there was a problem in that the test consequently required a troublesome operation. In addition, in order to make a detailed test, it was the only way to connect a measuring equipment to each display unit 1, and there was also a problem in that the construction of the testing equipment becomes complex and bulky.